来自《耶鲁大学开放课程：基础物理 第9集 第9集》

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  it's a tiny mass for this section.
  这一小块的质量很小
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  But what is the R^2 ?
  R^2 是多少呢
• 
  
  If I'm planning to rotate it around that point,
  如果我要将圆环绕着这个点转动
• 
  
  I plan to rotate around that point,
  我将圆环绕着圆心转动
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  it is the same R^2 for everybody.
  对每一小块来说 R^2 相同
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  So, you can pull it out of the summation
  那么你可以将它从整体中取出
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  and you just get M·R^2.
  就得到 M·R^2
• 
  
  So, this is a very easy problem.
  这是个很简单的问题
• 
  
  This is as easy as this problem,
  就像这个问题一样容易
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  but you clearly had only one mass
  那就是只有一个物体的情况
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  and all of it was at distance R from the point of rotation,
  它与旋转中心的距离为 R
• 
  
  it's M·R^2.
  答案也是 M·R^2